Steam turbines are commonly used to drive electrical generators in power plants. A typical steam turbine is a massive yet intricate piece of machinery that must be started up in a controlled manner in order to protect the many turbine components from damage from stresses and distortion that would result from immediate exposure to high temperature and pressure steam. One part of the turbine startup process is the prewarming procedure, which includes the steps of steam chest warming and turbine rotor prewarming. The chest warming procedure warms the control valve steam distribution chest to the stop valves; this step is accomplished before the control valves are opened during turbine acceleration and loading. The goal of the rotor prewarming process is to raise rotor bore temperatures without exceeding allowable rotor stress limits; rotor stress is controlled by bringing temperatures of critical rotor locations to specified values before the turbine is accelerated to its nominal operating speed.
When a steam turbine is being started up from a cold-shutdown condition, necessary auxiliary systems (such as lubrication oil, auxiliary steam systems, and the like) are placed in service. The turbine is placed on a turning gear, which keeps the turbine spinning slowly (e.g., 2-3 rpm) during the prewarming procedure. The steam chest of the turbine must be warmed in a manner such that differential temperatures between inner and outer walls of the steam chest do not cause excessive stress in the metal and such that the turbine steam control valve(s) are warmed prior to the unit being exposed to nominal steam system pressures and temperatures.
In prewarming the rotating components of the turbine, small amounts of steam are admitted to the high pressure side of the turbine to cause the turbine rotor to warm up, both through direct exposure to the steam and conduction of heat through turbine components, such as the rotor shaft: During the heating process, condensate from the steam admitted to the turbine must be drained off to avoid buildup of liquid in the turbine housing to prevent the turbine buckets from passing through water thereby causing cavitation damage. This procedure is continued until specified temperatures are reached on key turbine components and the steam pressure in the turbine reaches a selected level, at which time the turbine is ready to be accelerated and loaded.
It is desirable from an operational standpoint to complete the prewarming procedure in the shortest time consistent with turbine limitations such as allowable rotor stress. The prewarming procedure is typically accomplished by operators manually checking various indications of critical turbine parameters, such as rotor temperatures and stresses, and then implementing actions (such as opening or closing the steam supply valve) to increase or decrease, as appropriate, the rate of the turbine warm-up. There are several constraints on the rapidity with which the turbine can be warmed, however, such as the amount of steam admitted should not cause the turbine to "roll-off" the turning gear and accelerate prematurely; rotor and steam chest stress limits cannot be exceeded, and the turbine pressures and temperatures must be maintained within specified limits. In some situations, however, such as when there is an immediate need to have the turbine-generator in service, it is necessary to complete the prewarming procedure more rapidly than normal. In such circumstances, the operator must optimize the procedure by accepting some deviation from normal warm-up procedures, such as increasing the steam flow and allowing the turbine to periodically "roll off" the turning gear while ensuring that the turbine is not damaged by premature acceleration.
Efficiency and ease of operation make it advantageous to automate as many turbine operating control procedures as possible. There are, however, few good models of turbine operation at low temperatures and pressures, and thus traditional control oriented approaches to automation are not readily adapted to control the turbine prewarming procedure.
It is therefore an object of this invention to provide an automated method of prewarming a steam turbine.
A further object of this invention is to provide a turbine operation method employing fuzzy logic to generate signals to control the prewarming procedure for a steam turbine.
A still further object of this invention is to provide an automated prewarming procedure for a steam turbine that allows the operator to select a warm-up profile ranging between a slow warm-up rate and a fast warm-up rate.